1. Field of the Invention
The present invention relates to cushions, and in particular to seat cushions having an array of individual, expandable, fluid-filled cushioning cells for use by persons confined to wheelchairs and the like.
2. Description of the Related Art
In the United States alone, more than 247,000 individuals have complete or partial paralysis and more than 600,000 nursing home residents use wheelchairs. Many of these people require the use of a pressure-reducing cushion to minimize the risk of sitting-induced pressure ulcers. The prevalence of pressure ulcers among all nursing home residents is estimated between 7% and 23%. The incidence rate among other populations with mobility impairments is even higher; it has been estimated that between 50% and 80% of persons with spinal cord injury will develop a pressure ulcer. Even the lowest estimates indicate that pressure ulcers present a significant health care problem.
Pressure ulcers/sores are extremely dangerous and difficult to cure. These pressure sores, or decubitus ulcers, typically form in areas where bony prominences exist, such as the ischia, heels, elbows, ears and shoulders. Typically, when sitting, much of the individual's weight concentrates in the regions of the ischia, that is, at the bony prominences of the buttocks, and unless frequent movement occurs, the flow of blood to the skin tissue in these regions decreases to the point that the tissue breaks down. This problem is well known and many forms of cushions are especially designed for wheelchairs for reducing the concentration of weight in the region of the ischia. These cushions generally seek to distribute the user's weight more uniformly over a larger area of the buttocks.
Another area where pressure ulcers occur is in the trochanter area. Both cushions and bases for the cushions are often shaped so that pressure is relieved on the ischia and the trochanters. A significant problem with wheelchair-type cushions is stabilization of the user so that he has a feeling of security when sitting in the wheelchair.
Conventional cushioning devices for supporting the human body, such as the typical mattress, seat cushion or padded back rest, do not distribute the weight of the supported body evenly over the area of the body that is in contact with the cushioning device. For example, in the case of a mattress, the buttocks or hips, and likewise the shoulders, sink further into the mattress than the lumbar region of the back. Since most conventional cushioning devices exert a supporting force that is proportional to the amount they are deflected, those portions of the body which sink deepest into the cushioning device experience a resisting force per unit area that is considerably greater than those body portions that deflect the cushioning device only slightly. For those individuals who are confined to beds or wheelchairs for extended periods of time, the unequal distribution of supporting forces deforms the vascular system and reduces blood flow, which can lead to extreme discomfort and can even be debilitating in the sense that bed sores often develop at the skin areas where the supporting force is greatest.
While cushions which derive their cushioning properties from inner springs or foam material are quite common and inexpensive to manufacture, and offer good stability, they suffer the inability to distribute loads or develop restoring forces evenly to the object they are supporting. For example, expanded polymer foam of a resilient character, such as polyurethane, is a popular cushioning material for seating, and indeed finds widespread use in furniture and automotive seats. But resilient polymer foam does not produce the most desirable relationship between force and displacement. Far from this relationship being linear, it tends to be skewed, such that the force increases at a greater rate than the displacement, and this makes the material unusually stiff when an individual or object such as a bony prominence is deeply immersed in it. Thus, the region of the body that is most susceptible to injury receives the greatest resisting force per unit area, compounding the injury or increasing the risk thereof.
An effective cushion reduces pressure over bony prominences while providing stability and support, primarily through envelopment. The main types of wheelchair cushions can be described as fluid, compressive (elastic, viscoelastic), or suspension cushions. Fluid and fluid-like seat cushions achieve envelopment by accommodation of bony prominences and maintain the condition by virtue of their ability to dynamically adjust to changing loading conditions. However, the dynamic nature of fluid-filled cushions often leads to the undesirable characteristic of poor stability.
Cushions made from elastic materials such as high resilient foams must rely on pre-contouring to achieve envelopment. Such a cushion has no ability to dynamically adjust beyond the limits of the compliance of the material as defined by its material properties. That is, these cushions cannot change shape without a tendency to return to their original shape. When a person sits on the cushion both the cushion and the buttocks will deform until force equilibrium is reached. In the cushion, the counter forces will be greatest where there is the most deformation and least where there is low deformation as discussed above. Elastic cushions provide the advantage of enhanced stability due to the foam's tendency to hold its shape and, thus, hold the person in place. A fluid-like cushion instead changes its shape to accommodate changing load. The disadvantage of pre-contoured compared to fluid-like cushions is that the distribution of forces is sensitive to the relative match between the cushion and the buttock shapes, and to the positioning of the buttocks on the surface.
Cushions made from viscoelastic materials have a combination of elastic and fluid properties, giving such cushions some ability to reconfigure in a memoryless fashion and some ability to provide stability through resilience. An optimum balance of viscous and elastic response is a matter of personal preference and need, however, and may vary significantly from person to person.
Suspension cushions use the strategy of removal of material in the areas that commonly experience high pressure and use covers under tension to support these areas in a suspension-like manor. Suspension cushions remove material from the ischial area, and often the sacral area as well. The successful use of a suspension cushion also, as with a pre-contoured cushion, relies on a consistent positioning of the user on the surface.
Through clinical tests, it has been determined that one of the better methods of preventing the development of bed sores on patients is to support such persons on a series of flexible intercommunicated cells filled with a fluid such as air. Since the cells are intercommunicated all exert an equal supporting force against the engaged individual. Such an arrangement of cells is disclosed in U.S. Pat. No. 3,605,145.
Fluid cell cushions provide a uniform distribution of weight and thus provide good protection from the occurrence of pressure sores. These cushions have an array of closely-spaced air cells which project upwardly from a common base. Within the base the air cells communicate with each other, and thus all exist at the same internal pressure. Hence, each air cell exerts essentially the same restoring force against the buttocks, irrespective of the extent to which it is deflected. U.S. Pat. No. 4,541,136 shows a cellular cushion for use on wheelchairs.
The typical fluid cell cushion provides a highly displaceable surface which tends to float the user. While this reduces the incidence of pressure sores, it detracts from the stability one usually associates with a seating surface. Most of those confined to wheelchairs have little trouble adjusting to the decrease in stability, but for those who have skeletal deformities, particularly in the region of the pelvis and thighs, and for those who lack adequate strength in their muscles, lesser stability can be a source of anxiety.
The stability problem has been attacked by the use of shaped bases such as shown in Graebe, U.S. Pat. No. 4,953,913 and Jay, U.S. Pat. No. 4,726,624. These bases are generally used in conjunction with cushions. Graebe, U.S. Pat. No. 4,953,913 has been used in conjunction with a cellular cushion and a fabric cover. The stability problem also has been addressed in the cellular cushion by the use of zoned areas of inflation as shown in Graebe, U.S. Pat. No. 4,698,864, which shows a zoned cellular cushion with cells of varying height; and Graebe, U.S. Pat. No. 5,052,068, which shows another form of zoned cushions with cells of different heights. By varying the pressure between zones, one can accommodate for skeletal deformities, while still maintaining protection against pressure sores.
Graebe, U.S. Pat. No. 5,111,544, shows a cover for a zoned cellular cushion which keeps the cells from deflecting outwardly. This cover has a stretchable top, a skid resistant base, and a non-stretchable fabric side panel area.
Another problem with cushions of the prior art is the inability to accommodate individual shapes and sizes, or to be customized to provide greater support in areas needing it. One approach has been to employ cushions having separate adjustable zones, as discussed above, and such as described in U.S. Pat. No. 5,163,196.
Typically, a zoned cellular cushion has a separate filling stem and valve for each of its zones. The user opens the valve of each stem and introduces air into the zone for that stem, usually with a hand pump, and then releases the air from the zones until the desired posture is achieved. In a more sophisticated arrangement, a hose kit connects a single pump to a manifold which in turn is connected to the several valves through separate hoses. These hoses are fitted with separate hose clamps so that the air from the pump may be directed to the cells of the individual zones independently, and likewise the air can be released from them independently, all by manipulating the clamps. The hoses of the hose kit lie externally of the cushion and may become entangled in components of a wheelchair. Furthermore, by reason of their remote location, the hose clamps are difficult to manipulate. Also, such a design is not automatically adjustable, rather, may require repeated and cumbersome manual adjustment in order to achieve the desired level of comfort. In addition, while pressure may be varied from one zone to the next, all cells in a particular zone exert the same pressure, and fluid flow cannot be controlled between individual cells.
Other attempts to adjust cellular cushions include manually tying off cells in regions of the cushion, such as those regions supporting the ischia. Such efforts are cumbersome, however, and provide at best a trial and error solution to the problem.
Accordingly, an advance in the art could be realized if a cushion could be provided that offered the advantages of automatic contour adjustment, and that combined with optimum pressure-reducing and flexibility capabilities of air floatation, or cellular cushions, with stability closer to that of foam cushions.